Methods of forming thin-film photovoltaic devices with discontinuous passivation layers

ABSTRACT

In various embodiments, photovoltaic devices incorporate discontinuous passivation layers (i) disposed between a thin-film absorber layer and a partner layer, (ii) disposed between the partner layer and a front contact layer, and/or (iii) disposed between a back contact layer and the thin-film absorber layer.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/869,224, filed Sep. 29, 2015, now issued as U.S. Pat. No. 9,362,423,which is a continuation of U.S. patent application Ser. No. 14/492,693,filed Sep. 22, 2014, now issued as U.S. Pat. No. 9,178,082, which claimsthe benefit of and priority to U.S. Provisional Patent Application No.61/881,095, filed Sep. 23, 2013, the entire disclosure of each of whichis hereby incorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to thin-filmphotovoltaics, in particular to passivated thin-film photovoltaicmodules.

BACKGROUND

The use of thin-film photovoltaic (PV) devices based on amorphous Si(a-Si), cadmium telluride (CdTe), or copper indium gallium selenide(CuIn_(x)Ga_(1−x)Se₂ or CIGS) is becoming more widespread due tocontinued enhancements in cell efficiency, which are coupled withdecreasing costs. However, as in crystalline-silicon PV devices,charge-carrier recombination at interfaces within the cell or at exposedsurfaces of the cell can reduce cell efficiency via charge carrierlosses. Reducing charge-carrier recombination, therefore, canbeneficially increase the open circuit voltage, short circuit current,and efficiency of thin-film PV devices. While carrier recombination maybe reduced at exposed surfaces of PV devices through the use ofpassivating insulating layers (e.g., thermally grown silicon dioxidelayers on Si PV devices), such insulating layers block current flow andthus may not be utilized within the PV cell itself, or carrier transportwithin the PV cell will be disrupted or blocked. Thus, there is a needfor carrier-recombination techniques usable within the cell structuresof PV devices (e.g., at interfaces within the cell) that do notdeleteriously impact device efficiency.

SUMMARY

Embodiments of the present invention incorporate discontinuouspassivation layers within thin-film PV devices to reduce carrierrecombination. The passivation layers are discontinuous in the sensethat they define openings therethrough or that they consist essentiallyof a collection of “particles,” i.e., localized portions each discretefrom the others (and, thereby, once again, forming openings). Forexample, a substantially uniform passivation layer may be formed withinthe PV device and patterned to open holes therethrough or to separatethe passivation layer into multiple discrete portions (e.g., stripes).Alternatively, discrete particles of the passivating material may beformed directly within the device via, e.g., chemical bath deposition.

The passivation layers may be utilized to reduce or substantiallyeliminate electrically active surface and/or interface states within thePV device, and may thus be located (1) between the absorber layer andthe back contact, (2) between the absorber layer and the “partner layer”forming the electrical junction with the absorber layer, (3) between thepartner layer and the front contact layer, and/or (4) between otherlayers within the cell (for example, between CdTe and a material with aconduction band offset (e.g., ZnTe) utilized as an electron reflector,or even adjacent to internal lateral conducting layer in a multijunctioncell). The layers are discontinuous in order to enable sufficientelectrical contact between the layers at the interface being passivated.That is, continuous (e.g., unpatterned) passivation layers are generallynot utilized in accordance with embodiments of the present invention, asthey tend to result in deleterious increases in series resistance withinthe PV device.

Each passivation layer preferably includes or consists essentially of adielectric material. Exemplary materials include ZnS andhigh-dielectric-constant (i.e., “high-k”) dielectrics such as high-koxides. Particularly preferred examples include CaO, MgO, CaF₂, and LiF.The passivation layers also preferably meet the following criteria.First, the passivating material preferably has a high dielectricconstant greater than or approximately equal to 3.9, for example,greater than 10. Further, the passivation layer is generally chemically,thermally, and mechanically compatible with the subsequent processingsteps utilized to form and complete the PV device. The layers also arethermally robust, are substantially free of interdiffusion withadjoining layers after processing, and withstand high-temperatureambients while maintaining dielectric properties. The passivation layersare also formed with sufficiently low levels of film and interfacestress such that they exhibit excellent adhesion to adjoining layerswithout delamination. The layers are generally thermodynamically stableand thus do not react substantially with underlying material duringtheir formation and processing.

The passivation layers preferably have fairly large band gaps (e.g.,greater than 3 eV, greater than 5 eV, greater than 10 eV, and/or lessthan 15 eV) and band offsets to the conduction and valence band ofadjoining layers (e.g., the absorber layer, partner layer, front contactlayer, and/or back contact layer) of greater than 1 eV (and may be lessthan approximately 7.5 eV). The band offsets are preferably arranged inthe “type-I” or “straddling” arrangement such that the valence band ofthe passivation layer is lower in energy than the valence band ofadjacent layers and the conduction band of the passivation layer ishigher than the conduction band of adjacent layers. In some embodiments,the passivation layers include or consist essentially of an amorphousmaterial, and the material remains amorphous during and after subsequenthigh-temperature processing. The passivation layers are also preferablyeasy to pattern without damage to underlying layers. For example, thepassivation layers may have high solubilities in selective etchants thatdo not damage other layers of the PV device structure, and/or the layersmay exhibit high optical absorption of laser radiation that may beutilized to remove portions of the layers via, e.g., laser ablation orlaser drilling. In some embodiments, the passivation layers do notthemselves absorb large amounts of laser radiation; rather, all orsubstantially all of the light passes through the passivation layer andis absorbed into an underlying layer, leading to selective detachment ofthe portion of the passivation layer thereover (i.e., a laser “lift-off”process).

The thicknesses of the passivation layers may be, e.g., at leastapproximately 2 nm, at least approximately 5 nm, at least approximately10 nm, at least approximately 20 nm, at least approximately 40 nm, atleast approximately 50 nm, or even at least approximately 100 nm. Insome embodiments, the thickness of the passivation layers is no greaterthan 100 nm. The layers may be deposited by, e.g., physical vapordeposition methods such as e-beam evaporation, thermal evaporation, orsputtering, or by chemical vapor deposition (CVD) methods such asmetallorganic CVD, plasma-enhanced CVD, or atomic layer deposition. Thedeposited layers may be patterned via conventional photolithography andetch techniques to form, for example, a substantially periodic patternof openings. In other embodiments, the openings form a substantiallyrandom or semi-random pattern. The passivation layers may be patternedvia selective removal by laser ablation. The size and spacing of theopenings may vary at length scales of, e.g., less than 10 nm to tens orhundreds of microns or more with material of higher electrical quality(e.g., long carrier lifetime, long carrier diffusion length) enablingthe use of smaller openings and larger pitch between openings.Similarly, the shape of the contact pattern may be square, rectangular,circular, triangular, or of any suitable shape or polygon.Alternatively, the discontinuous passivation layers may be deposited indiscontinuous form, thus obviating the need for patterning. For example,localized particles of the passivating material may be deposited ontoone or more of the layers of the PV device structure. The size andspacing of the particles may vary at length scales of, e.g., tens orhundreds of nanometers up to tens of microns or more with material ofhigher electrical quality (e.g., long carrier lifetime, long carrierdiffusion length) enabling the use of larger particles and smaller pitchbetween particles. In other embodiments, a mask may be utilized toshadow portions of the PV device structure during deposition of thepassivating material, resulting in a discontinuous passivation layerdeposited only over regions where the mask is not present. The size andspacing of the shadowed features may vary at length scales of, e.g.,less than 10 nm to tens or hundreds of microns or more with material ofhigher electrical quality (e.g., long carrier lifetime, long carrierdiffusion length) enabling the use of smaller openings and larger pitchbetween openings.

In an aspect, embodiments of the invention feature a photovoltaic devicethat includes or consists essentially of a back contact layer, athin-film absorber layer, a partner layer, a front contact layer, andfirst, second, and/or third discontinuous passivation layers. The backcontact layer includes or consists essentially of a conductive material(e.g., a metal such as Mo). The thin-film absorber layer is disposedover and in electrical contact with the back contact layer. Thethin-film absorber layer has a doping polarity (i.e., n-type or p-type).The partner layer is disposed over and in electrical contact with thethin-film absorber layer. The partner layer has a doping polarityopposite that of the thin-film absorber layer, the partner layer andthin-film absorber layer thereby forming a p-n junction. The frontcontact layer is disposed over and in electrical contact with thepartner layer. The first discontinuous passivation layer, if present, isdisposed between the thin-film absorber layer and the partner layer, thepartner layer making electrical contact with the thin-film absorberlayer only through discontinuities in the first discontinuouspassivation layer. The second discontinuous passivation layer, ifpresent, is disposed between the partner layer and the front contactlayer, the front contact layer making electrical contact with thepartner layer only through discontinuities in the second discontinuouspassivation layer. The third discontinuous passivation layer, ifpresent, is disposed between the back contact layer and the thin-filmabsorber layer, the thin-film absorber layer making electrical contactwith the back contact layer only through discontinuities in the thirddiscontinuous passivation layer (and discontinuities in a discontinuousback reflector layer and/or a sodium-containing layer, if present).

Embodiments of the invention may include one or more of the following inany of a variety of different combinations. The front contact layer mayinclude or consist essentially of a transparent conductive oxide (e.g.,indium tin oxide). The back contact layer may include or consistessentially of molybdenum. The back contact layer may include or consistessentially of a sodium-containing conductive material (e.g., Mo:NaFand/or Mo:Na₂MoO₄). The absorber layer may include or consistessentially of amorphous silicon, CdTe, chalcopyrite (Cu(In,Ga)(S,Se)₂),and/or kesterite (Cu₂(Zn,Fe)Sn(S,Se)₄). The first, second, and/or thirddiscontinuous passivation layers may include or consist essentially ofan insulator having a dielectric constant greater than or approximatelyequal to 3.9. The first, second, and/or third discontinuous passivationlayers may include or consist essentially of an insulator having adielectric constant greater than or approximately equal to 10. Thefirst, second, and/or third discontinuous passivation layers may have aband gap exceeding 3 eV. The first, second, and/or third discontinuouspassivation layers may have a band offset to an adjoining layerexceeding 1 eV. The band offset may be a type-I band offset. The first,second, and/or third discontinuous passivation layers may include orconsist essentially of CaO, MgO, and/or ZnS. The device may include asodium-containing layer disposed between the back contact layer and thethin-film absorber layer. The sodium-containing layer may be continuousor discontinuous. Discontinuities in a discontinuous sodium-containinglayer may overlap partially or substantially entirely (i.e., besubstantially aligned) with discontinuities in a discontinuouspassivation layer and/or a discontinuous reflector layer. Alternatively,the discrete regions of a discontinuous sodium-containing layer maypartially or substantially entirely overlap the discontinuities in adiscontinuous passivation layer and/or a discontinuous reflector layer.The sodium-containing layer may include or consist essentially of NaFand/or Na₂Se. The device may include a discontinuous reflector layerdisposed between the back contact layer and the thin-film absorberlayer. The discontinuous reflector layer may reflect solar energypassing through the absorber layer back toward the absorber layer. Thediscontinuous reflector layer may include or consist essentially ofaluminum, silver, titanium dioxide, and/or zirconium nitride.

In another aspect, embodiments of the invention feature a method forforming a photovoltaic device. A thin-film absorber layer is formed overand in electrical contact with a back contact layer. The thin-filmabsorber layer has a doping polarity. The back contact layer includes orconsists essentially of a conductive material. A partner layer is formedover and in electrical contact with the thin-film absorber layer. Thepartner layer has a doping polarity opposite that of the thin-filmabsorber layer, the partner layer and thin-film absorber layer therebyforming a p-n junction. A front contact layer disposed over and inelectrical contact with the partner layer is formed. A firstdiscontinuous passivation layer, a second discontinuous passivationlayer, and/or a third discontinuous passivation layer is formed. Thefirst discontinuous passivation layer, if formed, is disposed betweenthe thin-film absorber layer and the partner layer, the partner layermaking electrical contact with the thin-film absorber layer only throughdiscontinuities in the first discontinuous passivation layer. The seconddiscontinuous passivation layer, if formed, is disposed between thepartner layer and the front contact layer, the front contact layermaking electrical contact with the partner layer only throughdiscontinuities in the second discontinuous passivation layer. The thirddiscontinuous passivation layer, if formed, is disposed between the backcontact layer and the thin-film absorber layer, the thin-film absorberlayer making electrical contact with the back contact layer only throughdiscontinuities in the third discontinuous passivation layer (anddiscontinuities in a discontinuous back reflector layer and/or asodium-containing layer, if present).

Embodiments of the invention may include one or more of the following inany of a variety of different combinations. The first discontinuouspassivation layer may be formed by a process including or consistingessentially of forming a passivation layer over the thin-film absorberlayer, and patterning the passivation layer to form the firstdiscontinuous passivation layer and reveal portions of the thin-filmabsorber layer through discontinuities in the first discontinuouspassivation layer. The partner layer may make electrical contact withthe thin-film absorber layer through the discontinuities in the firstdiscontinuous passivation layer. The first discontinuous passivationlayer may be formed by a process including or consisting essentially ofdepositing discrete particles of a passivating material over thethin-film absorber layer, regions between the discrete particles beingthe discontinuities in the first discontinuous passivation layer. Thefirst discontinuous passivation layer may be formed by a processincluding or consisting essentially of disposing a mask over thethin-film absorber layer, only portions of the thin-film absorber layerbeing revealed through openings in the mask, and depositing apassivating material over the mask to form discrete portions of thepassivating material through the openings in the mask, regions betweenthe discrete portions being the discontinuities in the firstdiscontinuous passivation layer.

The second discontinuous passivation layer may be formed by a processincluding or consisting essentially of forming a passivation layer overthe partner layer, and patterning the passivation layer to form thesecond discontinuous passivation layer and reveal portions of thepartner layer through discontinuities in the second discontinuouspassivation layer. The front contact layer may make electrical contactwith the partner layer through the discontinuities in the seconddiscontinuous passivation layer. The second discontinuous passivationlayer may be formed by a process including or consisting essentially ofdepositing discrete particles of a passivating material over the partnerlayer, regions between the discrete particles being the discontinuitiesin the second discontinuous passivation layer. The second discontinuouspassivation layer may be formed by a process including or consistingessentially of disposing a mask over the partner layer, only portions ofthe partner layer being revealed through openings in the mask, anddepositing a passivating material over the mask to form discreteportions of the passivating material through the openings in the mask,regions between the discrete portions being the discontinuities in thesecond discontinuous passivation layer.

The third discontinuous passivation layer may be formed by a processincluding or consisting essentially of forming a passivation layer overthe back contact layer, and patterning the passivation layer to form thethird discontinuous passivation layer and reveal portions of the backcontact layer through discontinuities in the third discontinuouspassivation layer. The thin-film absorber layer may make electricalcontact with the back contact layer through discontinuities in the thirddiscontinuous passivation layer. The third discontinuous passivationlayer may be formed by a process including or consisting essentially ofdepositing discrete particles of a passivating material over the backcontact layer, regions between the discrete particles being thediscontinuities in the third discontinuous passivation layer. The thirddiscontinuous passivation layer may be formed by a process including orconsisting essentially of disposing a mask over the back contact layer,only portions of the back contact layer being revealed through openingsin the mask, and depositing a passivating material over the mask to formdiscrete portions of the passivating material through the openings inthe mask, regions between the discrete portions being thediscontinuities in the third discontinuous passivation layer.

A sodium-containing layer may be formed over the back contact layerprior to forming the thin-film absorber layer. The sodium-containinglayer may include or consist essentially of NaF and/or Na₂Se. Adiscontinuous back reflector may be formed over the back contact layerprior to forming the thin-film absorber layer. The discontinuousreflector layer may reflect solar energy passing through the absorberlayer back toward the absorber layer. The discontinuous reflector layermay include or consist essentially of aluminum, silver, titaniumdioxide, and/or zirconium nitride. Forming the discontinuous backreflector may include or consist essentially of depositing discreteparticles of a back-reflector material over the back contact layer,regions between the discrete particles being the discontinuities in thediscontinuous back reflector. Forming the discontinuous back reflectormay include or consist essentially of disposing a mask over the backcontact layer, only portions of the back contact layer being revealedthrough openings in the mask, and depositing a back-reflector materialover the mask to form discrete portions of the back-reflector materialthrough the openings in the mask, regions between the discrete portionsbeing the discontinuities in the discontinuous back reflector. Formingthe discontinuous back reflector may include or consist essentially offorming a layer of back-reflector material over the back contact layer,and patterning the layer of back-reflector material to form thediscontinuous back reflector and reveal portions of the back contactlayer through discontinuities in the discontinuous back reflector. Thethin-film absorber layer may make electrical contact with the backcontact layer through the discontinuities in the discontinuous backreflector. The third discontinuous passivation layer may be formed afterthe discontinuous back reflector is formed, and at least somediscontinuities in the discontinuous back reflector layer may overlapwith discontinuities in the third discontinuous passivation layer.

The discontinuous back reflector and the third discontinuous passivationlayer may be formed by a process including or consisting essentially offorming a layer of back-reflector material over the back contact layer,forming a passivation layer over the layer of back-reflector material,and thereafter, patterning the passivation layer and the layer ofback-reflector material to form the third discontinuous passivationlayer and, thereunder, the discontinuous back reflector, therebyrevealing portions of the back contact layer through discontinuities inthe third discontinuous passivation layer and discontinuities in thediscontinuous back reflector. The thin-film absorber layer may makeelectrical contact with the back contact layer through discontinuitiesin the third discontinuous passivation layer and discontinuities in thediscontinuous back reflector (which may be substantially aligned withthe discontinuities in the third discontinuous passivation layer). Asodium-containing layer may be formed over the back contact layer (e.g.,over the third discontinuous passivation layer and over thediscontinuous back reflector layer) prior to forming the thin-filmabsorber layer. The sodium-containing layer may include or consistessentially of NaF and/or Na₂Se.

The front contact layer may include or consist essentially of atransparent conductive oxide. The back contact layer may include orconsist essentially of molybdenum. The back contact layer may include orconsist essentially of a sodium-containing conductive material (e.g.,Mo:NaF and/or Mo:Na₂MoO₄). The absorber layer may include or consistessentially of amorphous silicon, CdTe, chalcopyrite (Cu(In,Ga)(S,Se)₂),and/or kesterite (Cu₂(Zn,Fe)Sn(S,Se)₄). The first, second, and/or thirddiscontinuous passivation layers may include or consist essentially ofan insulator having a dielectric constant greater than or approximatelyequal to 3.9. The first, second, and/or third discontinuous passivationlayers may include or consist essentially of an insulator having adielectric constant greater than or approximately equal to 10. Thefirst, second, and/or third discontinuous passivation layers may have aband gap exceeding 3 eV. The first, second, and/or third discontinuouspassivation layers may have a band offset to an adjoining layer (e.g., alayer is direct contact with the passivation layer) exceeding 1 eV. Theband offset may be a type-I band offset. The first, second, and/or thirddiscontinuous passivation layers may include or consist essentially ofCaO, MgO, and/or ZnS.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately” and “substantially” mean±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic cross-section of a portion of a photovoltaicdevice incorporating a discontinuous passivation layer in accordancewith various embodiments of the invention;

FIG. 2 is a schematic cross-section of a portion of a photovoltaicdevice incorporating a discontinuous passivation layer and asodium-containing layer in accordance with various embodiments of theinvention;

FIG. 3A is a schematic plan view of the discontinuous passivation layerof FIG. 1 in accordance with various embodiments of the invention;

FIGS. 3B-3D are schematic plan views of the structure of FIG. 3A duringfabrication thereof in accordance with various embodiments of theinvention;

FIG. 4 is a schematic plan view of the discontinuous passivation layerof FIG. 1 in accordance with various other embodiments of the invention;

FIG. 5A is a schematic cross-section of a portion of a photovoltaicdevice incorporating a discontinuous passivation layer and adiscontinuous reflector layer in accordance with various embodiments ofthe invention;

FIG. 5B is a schematic cross-section of a portion of a photovoltaicdevice incorporating a discontinuous passivation layer, a discontinuousreflector layer, and a sodium-containing layer in accordance withvarious embodiments of the invention;

FIGS. 6A-6D are schematic plan views of the discontinuous passivationlayer and discontinuous reflector layer of FIG. 5A during fabricationthereof in accordance with various embodiments of the invention;

FIGS. 7A and 7B are schematic cross-sections of photovoltaic devicesincorporating discontinuous passivation layers in accordance withvarious embodiments of the invention;

FIG. 8A is a schematic plan view of the discontinuous passivation layerof FIG. 7A in accordance with various embodiments of the invention;

FIGS. 8B-8D are schematic plan views of the structure of FIG. 8A duringfabrication thereof in accordance with various embodiments of theinvention;

FIG. 9 is a schematic plan view of the discontinuous passivation layerof FIG. 7A in accordance with various other embodiments of theinvention;

FIG. 10 is a schematic cross-section of a portion of a photovoltaicdevice incorporating two discontinuous passivation layers in accordancewith various embodiments of the invention;

FIGS. 11A-11G are schematic plan views of the structure of FIG. 10during fabrication thereof in accordance with various embodiments of theinvention;

FIG. 12A is a schematic cross-section of a portion of a photovoltaicdevice incorporating a discontinuous passivation layer in accordancewith various embodiments of the invention; and

FIG. 12B is a schematic cross-section of a portion of a photovoltaicdevice incorporating two discontinuous passivation layers in accordancewith various embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 depicts an exemplary embodiment of the present invention in whicha thin-film PV device 100 incorporates a discontinuous passivation layer110 disposed between a back contact 120 and a thin-film absorber layer130. The back contact layer 120 may include or consist of, for example,a highly electrically conductive material such as a metal. In someembodiments the back contact layer 120 includes or consists essentiallyof a refractory metal such as molybdenum (Mo). In some embodiments ofthe invention, the thin-film PV device 100 also incorporates asodium-containing layer (as detailed below); in some embodiments, theback contact layer 120 itself contains sodium. For example, the backcontact layer 120 may include or consist essentially of Mo:NaF orMo:Na₂MoO₄. FIG. 1 depicts an embodiment in which the back contact 120is disposed on a substrate 140 (e.g., soda lime glass), but “superstate”embodiments, in which the “substrate” is disposed above the absorberlayer 130 (and the remaining layers of the device) are included in thescope of the present invention. Although not depicted in its entirety inFIG. 1, the thin-film PV device 100 itself includes one or more p-nand/or p-i-n junctions (i.e., homojunctions and/or heterojunctions), andis fabricated from a-Si, CdTe, or a chalcopyrite (Cu(In,Ga)(S,Se)₂) suchas copper indium gallium selenide (CIGS) or a kesterite(Cu₂(Zn,Fe)Sn(S,Se)₄) such as copper zinc tin sulfide (CZTS). Forexample, for a PV device 100 in which the absorber layer 130 includes orconsists essentially of CIGS, the device 100 may include a junctionformed via the incorporation of a CdS layer disposed over the absorberlayer 130, as discussed below and as illustrated in subsequent figures.Thus, it is to be understood that the PV devices illustrated herein mayonly show portions of the device relevant to the particular placement ofdiscontinuous passivation layers in accordance with embodiments of theinvention and may therefore incorporate additional layers neither shownnor described.

FIG. 2 depicts a thin-film PV device 200 similar to that depicted inFIG. 1, except for the presence of a sodium-containing layer 210 betweenthe passivation layer 110 and the absorber layer 130. In someembodiments, the presence of a sodium-containing layer 210 (which mayinclude or consist essentially of, e.g., NaF and/or Na₂Se) improves theefficiency of the thin-film PV device 200. The sodium-containing layer210 may supply sodium to the absorber layer 130 during formationthereof; additional sodium may be supplied by the substrate 140—suchsodium may diffuse through the back contact layer 120 to the absorberlayer 130. Sodium may also be introduced into the PV device 200 in otherways, including as part of the back contact layer 120, or during orafter formation of the thin-film absorber layer 130. Thesodium-containing layer 210 (and/or other sodium-containing layersdescribed herein) may be continuous (as shown) or discontinuous.Discontinuities in a discontinuous sodium-containing layer 210 mayoverlap partially or substantially entirely (i.e., be substantiallyaligned) with discontinuities in a discontinuous passivation layerand/or a discontinuous reflector layer. Alternatively, the discreteregions of a discontinuous sodium-containing layer 210 may partially orsubstantially entirely overlap the discontinuities in a discontinuouspassivation layer and/or a discontinuous reflector layer.

FIG. 3A depicts a plan view of the discontinuous passivation layer 110of FIG. 1 after its formation, and FIGS. 3B-3D depict an exemplaryprocess for fabricating the discontinuous passivation layer 110. FIG. 3Bdepicts the back contact layer 120 (which may itself be formed by, e.g.,sputtering upon the substrate 140) prior to the formation of thepassivation layer 110. As shown in FIGS. 3C and 3D, the passivationlayer 110 may be deposited over the back contact layer 120 as acontinuous film (FIG. 3C) and subsequently patterned to form openingsthat expose portions of the back contact layer 120 (FIG. 3D). Thethin-film absorber layer 130 may then be formed over the discontinuouspassivation layer 110 and make contact with the exposed portions of theback contact layer 120. In other embodiments, a mask is disposed overthe back contact layer 120 such that only portions of the back contactlayer 120 are revealed through openings in the mask. The passivationlayer 110 may then be deposited over the mask to form discrete portionsthereof through the openings in the mask, the regions between thediscrete portions being the discontinuities in the passivation layer110. FIG. 4 is a plan view of another exemplary embodiment of theinvention that incorporates a discontinuous passivation layer 110. Asshown, the passivation layer 110 has been patterned to form multipleelongated stripes over the back contact layer 120, which is exposedbetween the passivating stripes. As described above, the absorber layer130 may be formed over the illustrated structure and make electricalcontact with the exposed portions of the back contact layer 120.

FIGS. 5A and 5B depict exemplary thin-film PV devices 500, 510 inaccordance with embodiments of the present invention that incorporate aback optical reflector layer 520 between the discontinuous passivationlayer 110 and the back contact 120. The back reflector layer 520 mayinclude or consist essentially of a metal (e.g., aluminum) or anothermaterial (e.g., TiO₂) reflective to solar energy. The back reflector 520reflects solar energy passing through the absorber layer 130 back to theabsorber layer 130, thereby increasing the probability of absorption andthe efficiency of the PV device. Materials such as aluminum may not formohmic contacts with absorber layers including or consisting essentiallyof CIGS, and thus PV devices 500, 510 each incorporate a discontinuousback reflector layer 520 (e.g., patterned like the passivation layer110) so that the absorber layer 130 may make electrical contact directlywith the back contact layer 120. As shown in FIG. 5B, PV device 510includes the sodium-containing layer 210 described above while PV device500 omits this layer.

FIGS. 6A-6D depict portions of an exemplary process, in plan view, forfabricating part of the PV device 500 depicted in FIG. 5A. FIG. 6Adepicts the back contact layer 120 (which may itself be formed by, e.g.,sputtering upon the substrate 140) prior to the formation of the backreflector layer 520. As shown in FIG. 6B, the back reflector layer 520may be deposited over the back contact layer 120 either in patternedform (e.g., as a collection of particles or segments) or as a continuouslayer that is subsequently patterned to expose portions of theunderlying back contact layer 120. The back reflector layer 520 may evenbe deposited over a mask having openings where the portions of the backreflector layer 520 are desired. The passivation layer 110 may bedeposited over the discontinuous back reflector layer 520 (FIG. 6C) andalso patterned to reveal the underlying back contact layer 120 (FIG.6D). (In other embodiments, a mask is disposed over the discontinuousback reflector layer 520 such that all or portions of the discontinuousback reflector layer 520 are revealed through openings in the mask. Thepassivation layer 110 may then be deposited over the mask to formdiscrete portions thereof through the openings in the mask, the regionsbetween the discrete portions being the discontinuities in thepassivation layer 110.) As shown, at least some of the discontinuities(e.g., holes) in the passivation layer 110 and the back reflector layer520 overlap, thereby revealing portions of the back reflector layer 120through both the discontinuous back reflector layer 520 and thediscontinuous passivation layer 110. The thin-film absorber layer 130may then be formed over the discontinuous passivation layer 110 and makeelectrical contact with the back contact layer 120 through thediscontinuities, as shown in FIG. 5A.

FIG. 7A depicts an exemplary PV device 700 in accordance with variousembodiments of the present invention, in which the discontinuouspassivation layer 110 is formed between the thin-film absorber layer 130and a “partner layer” 710 forming the electrical p-n junction with theabsorber layer. For example, if the absorber layer 130 exhibits p-typedoping, then the partner layer 710 exhibits n-type doping to form therequisite p-n junction. The partner layer 710 may include or consistessentially of the same material as the absorber layer 130 (therebyforming a homojunction) or a different material (thereby forming aheterojunction). FIG. 7A also depicts a front contact layer 720 utilizedto contact the top of the thin-film PV device 700. In embodiments inwhich solar energy illuminates the absorber layer 130 through the frontcontact layer 720, the front contact layer is preferably at leastsubstantially transparent to solar energy (or one or more portions ofthe solar spectrum). Thus, the front contact layer 720 may include orconsist essentially of, e.g., a transparent conductive oxide such asindium tin oxide or (B,Al,Ga,In)₂O₃:ZnO. In some embodiments of theinvention, as shown for PV device 730 of FIG. 7B, in order to reducecarrier recombination at the interface between the partner layer 710 andthe front contact layer 720, the discontinuous passivation layer 110 isformed between the partner layer 710 and the front contact layer 720.

FIG. 8A depicts a plan view of the discontinuous passivation layer 110of FIG. 7A after its formation, and FIGS. 8B-8D depict an exemplaryprocess for fabricating the discontinuous passivation layer 110. FIG. 8Bdepicts the absorber layer 130 prior to the formation of the passivationlayer 110. As shown in FIGS. 8C and 8D, the passivation layer 110 may bedeposited over the absorber layer 130 as a continuous film (FIG. 8C) andsubsequently patterned to form openings that expose portions of theabsorber layer 130 (FIG. 8D). The partner layer 710 may then be formedover the discontinuous passivation layer 110 and make contact with theexposed portions of the absorber layer 130, as shown in FIG. 7A. FIG. 9is a plan view of another exemplary embodiment of the invention thatincorporates a discontinuous passivation layer 110. As shown, thepassivation layer 110 has been patterned to form multiple elongatedstripes over the absorber layer 130, which is exposed between thepassivating stripes. As described above, the partner layer 710 may beformed over the illustrated structure and make electrical contact withthe exposed portions of the absorber layer 130.

Embodiments of the invention incorporate multiple differentdiscontinuous passivation layers 110 disposed at different locationswithin the PV device structure. FIG. 10 depicts the cross-section of anexemplary PV device 1000 that incorporates a first discontinuouspassivation layer 110-1 between the thin-film absorber layer 130 and thepartner layer 710, as well as a second discontinuous passivation layer110-2 between the partner layer 710 and the front contact layer 720.Although not depicted in FIG. 10, such structures may even include adiscontinuous passivation layer (and/or back reflector layer) disposedbetween the thin-film absorber layer 130 and the back contact layer 120(as shown in FIGS. 1, 5A, and/or 5B) in addition to the two passivationlayers 110-1, 110-2 (or instead of one or the other of them). Thepatterns of the individual passivation layers 110 need not have the samegeometry, feature size, or pitch.

FIGS. 11A-11G depict an exemplary process for fabricating thediscontinuous passivation layers 110-1, 110-2 depicted in FIG. 10. FIG.11A depicts the absorber layer 130 prior to the formation of the firstpassivation layer 110-1. As shown in FIGS. 11B and 11C, the firstpassivation layer 110-1 may be deposited over the absorber layer 130 asa continuous film (FIG. 11B) and subsequently patterned to form openingsthat expose portions of the absorber layer 130 (FIG. 11C). (In otherembodiments, a mask is disposed over the absorber layer 130 such thatportions of the absorber layer 130 are revealed through openings in themask. The first passivation layer 110-1 may then be deposited over themask to form discrete portions thereof through the openings in the mask,the regions between the discrete portions being the discontinuities inthe first passivation layer 110-1.) The partner layer 710 may then beformed over the first discontinuous passivation layer 110-1 and makecontact with the exposed portions of the absorber layer 130, as shown inFIG. 11D (and FIG. 10). As shown in FIGS. 11E and 11F, the secondpassivation layer 110-2 may be deposited over the partner layer 710 as acontinuous film (FIG. 11E) and subsequently patterned to form openingsthat expose portions of the partner layer 710 (FIG. 11F). (In otherembodiments, a mask is disposed over the partner layer 710 such that allor portions of the partner layer 710 are revealed through openings inthe mask. The second passivation layer 110-2 may then be deposited overthe mask to form discrete portions thereof through the openings in themask, the regions between the discrete portions being thediscontinuities in the second passivation layer 110-2.) The frontcontact layer 720 may then be formed over the second discontinuouspassivation layer 110-2 and make contact with the exposed portions ofthe partner layer 710, as shown in FIG. 11G (and FIG. 10). The patternsof the individual passivation layers 110-1, 110-2 need not have the samegeometry, feature size, or pitch.

FIGS. 12A and 12B depict exemplary PV devices 1200, 1210 in accordancewith embodiments of the present invention in which the discontinuouspassivation layer 110 is formed (e.g., deposited) as a collection ofparticles or other discrete portions, rather than deposited as a uniformlayer and subsequently patterned. As shown, in PV device 1200 thepassivation layer 110 is deposited as a collection of particles on thethin-film absorber layer 130 and subsequently covered with the partnerlayer 710, which makes electrical contact with the absorber layer 130 inthe regions between the particles (i.e., the “discontinuities” in thediscontinuous passivation layer 110). As shown in FIG. 12B, PV device1210 incorporates a first passivation layer 110-1 similar or identicalto the passivation layer 110 in PV device 1200, as well as a secondpassivation layer 110-2 formed over the partner layer 710 andsubsequently patterned prior to formation of the front contact layer720. In order to form the discontinuous passivation layers 110 in PVdevices 1200, 1210 as a collection of discrete portions, the passivationlayer material may be deposited over the device structure through a maskhaving openings where the passivation layer 110 is desired; afterformation of the resulting discrete portions, the mask is removed andthe additional layers of the PV device structure are formed.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. A method for forming a photovoltaic deviceconfigured for top illumination by solar energy, the method comprising:providing a back contact layer comprising a conductive material; forminga discontinuous back reflector over the back contact layer; forming athin-film absorber layer over and in electrical contact with the backcontact layer, the thin-film absorber layer (i) having a doping polarityand (ii) comprising CdTe, chalcopyrite, or kesterite, wherein (a) theabsorber layer makes direct electrical contact to the back contact layerthrough discontinuities in the discontinuous back reflector, and (b) thediscontinuous back reflector is in contact with, but does not form anohmic contact to, the absorber layer; forming a partner layer over andin electrical contact with the thin-film absorber layer, the partnerlayer having a doping polarity opposite that of the thin-film absorberlayer, the partner layer and thin-film absorber layer thereby forming ap-n junction; forming a front contact layer disposed over and inelectrical contact with the partner layer, wherein (i) the front contactlayer comprises a transparent conductive oxide, and (ii) thediscontinuous back reflector is positioned to reflect solar energypassing through the front contact layer and the absorber layer backtoward the absorber layer; and forming a discontinuous passivation layerdisposed between the partner layer and the front contact layer, thefront contact layer making electrical contact with the partner layeronly through discontinuities in the discontinuous passivation layer; andproviding a transparent superstrate over the front contact layer, thetransparent superstrate being electrically insulating, wherein, betweenthe discontinuities in the discontinuous passivation layer, the frontcontact layer extends, as a continuous layer, over an entirety of thediscontinuous passivation layer.
 2. The method of claim 1, wherein thepartner layer and the thin-film absorber layer comprise the samematerial.
 3. The method of claim 1, wherein the discontinuouspassivation layer is formed by a process comprising: forming acontinuous passivation layer over the partner layer; and patterning thecontinuous passivation layer to form the discontinuous passivation layerand reveal portions of the partner layer through discontinuities in thediscontinuous passivation layer, wherein the front contact layer makeselectrical contact with the partner layer through the discontinuities inthe second discontinuous passivation layer.
 4. The method of claim 1,wherein the discontinuous passivation layer is formed by a processcomprising depositing discrete particles of a passivating material overthe partner layer, regions between the discrete particles being thediscontinuities in the discontinuous passivation layer.
 5. The method ofclaim 1, wherein the discontinuous passivation layer is formed by aprocess comprising: disposing a mask over the partner layer, onlyportions of the partner layer being revealed through openings in themask; and depositing a passivating material over the mask to formdiscrete portions of the passivating material through the openings inthe mask, regions between the discrete portions being thediscontinuities in the discontinuous passivation layer.
 6. The method ofclaim 1, wherein the discontinuous reflector layer comprises at leastone of aluminum, silver, titanium dioxide, or zirconium nitride.
 7. Themethod of claim 1, wherein the back contact layer comprises molybdenum.8. The method of claim 1, wherein the back contact layer comprises asodium-containing conductive material.
 9. The method of claim 8, whereinthe sodium-containing conductive material comprises at least one ofMo:NaF or Mo:Na₂MoO₄.
 10. The method of claim 1, wherein the backcontact layer is disposed on a substrate, the substrate (i) notcontacting the thin-film absorber layer and (ii) being electricallyinsulating.
 11. The method of claim 10, further comprising, duringformation of the thin-film absorber layer, diffusing sodium from thesubstrate into the thin-film absorber layer.
 12. The method of claim 1,wherein the transparent superstrate comprises glass.
 13. The method ofclaim 1, wherein the discontinuous back reflector is metallic.
 14. Themethod of claim 1, wherein the discontinuous back reflector isnon-metallic.
 15. A method for forming a photovoltaic device configuredfor top illumination by solar energy, the method comprising: providing ametallic back contact layer; forming a discontinuous back reflector overthe back contact layer; forming a thin-film absorber layer over and inelectrical contact with the back contact layer, the thin-film absorberlayer (i) having a doping polarity and (ii) comprising CdTe,chalcopyrite, or kesterite, wherein (a) the absorber layer makes directelectrical contact to the back contact layer through discontinuities inthe discontinuous back reflector, and (b) the discontinuous backreflector is in contact with, but does not form an ohmic contact to, theabsorber layer; forming a partner layer over and in electrical contactwith the thin-film absorber layer, the partner layer having a dopingpolarity opposite that of the thin-film absorber layer, the partnerlayer and thin-film absorber layer thereby forming a p-n junction;forming a front contact layer disposed over and in electrical contactwith the partner layer, the front contact layer being disposed over anentirety of the partner layer, wherein (i) the front contact layercomprises a transparent conductive oxide, and (ii) the discontinuousback reflector is positioned to reflect solar energy passing through thefront contact layer and the absorber layer back toward the absorberlayer; forming a discontinuous passivation layer disposed between theback contact layer and the thin-film absorber layer, the thin-filmabsorber layer making electrical contact with the back contact layeronly through discontinuities in the discontinuous passivation layer; andproviding a transparent superstrate over the front contact layer, thetransparent superstrate being electrically insulating.
 16. The method ofclaim 15, wherein the back contact layer comprises at least one ofmolybdenum, Mo:NaF, or Mo:Na₂MoO₄.
 17. The method of claim 15, whereinthe partner layer and the thin-film absorber layer comprise the samematerial.
 18. The method of claim 15, wherein the discontinuouspassivation layer is formed by a process comprising: forming acontinuous passivation layer over the back contact layer; and patterningthe continuous passivation layer to form the discontinuous passivationlayer and reveal portions of the back contact layer throughdiscontinuities in the discontinuous passivation layer, wherein thethin-film absorber layer makes electrical contact with the back contactlayer through discontinuities in the discontinuous passivation layer.19. The method of claim 15, wherein the discontinuous passivation layeris formed by a process comprising depositing discrete particles of apassivating material over the back contact layer, regions between thediscrete particles being the discontinuities in the discontinuouspassivation layer.
 20. The method of claim 15, wherein the discontinuouspassivation layer is formed by a process comprising: disposing a maskover the back contact layer, only portions of the back contact layerbeing revealed through openings in the mask; and depositing apassivating material over the mask to form discrete portions of thepassivating material through the openings in the mask, regions betweenthe discrete portions being the discontinuities in the discontinuouspassivation layer.
 21. The method of claim 15, further comprising, priorto forming the thin-film absorber layer, forming a sodium-containinglayer over the discontinuous passivation layer.
 22. The method of claim21, wherein the sodium-containing layer comprises at least one of NaF orNa₂Se.
 23. The method of claim 15, wherein the discontinuous reflectorlayer comprises at least one of aluminum, silver, titanium dioxide, orzirconium nitride.
 24. The method of claim 15, wherein the thirddiscontinuous passivation layer is formed after the discontinuous backreflector is formed, at least some discontinuities in the discontinuousback reflector layer overlapping with discontinuities in thediscontinuous passivation layer.
 25. The method of claim 15, wherein thediscontinuous back reflector and the discontinuous passivation layer areformed by a process comprising: forming a continuous layer ofback-reflector material over the back contact layer; forming acontinuous passivation layer over the layer of back-reflector material;and thereafter, patterning the passivation layer and the layer ofback-reflector material to form the discontinuous passivation layer and,thereunder, the discontinuous back reflector, thereby revealing portionsof the back contact layer through discontinuities in the discontinuouspassivation layer and discontinuities in the discontinuous backreflector, wherein the thin-film absorber layer makes electrical contactwith the back contact layer through discontinuities in the discontinuouspassivation layer and discontinuities in the discontinuous backreflector.
 26. The method of claim 25, further comprising, prior toforming the thin-film absorber layer, forming a sodium-containing layerover the discontinuous passivation layer.
 27. The method of claim 15,wherein the back contact layer is disposed on a substrate, the substrate(i) not contacting the thin-film absorber layer and (ii) beingelectrically insulating.
 28. The method of claim 24, further comprising,during formation of the thin-film absorber layer, diffusing sodium fromthe substrate into the thin-film absorber layer.
 29. The method of claim15, wherein the transparent superstrate comprises glass.
 30. The methodof claim 15, wherein the discontinuous back reflector is metallic. 31.The method of claim 15, wherein the discontinuous back reflector isnon-metallic.